Semiconductor light sources such as light emitting diodes (LEDs) as a liquid crystal backlight or lighting device have become prevalent. FIG. 1 is a block diagram of a lighting device including an LED. A lighting device 500r includes an LED light source 502, a rectifying circuit 504, a smoothing condenser 506, and a switching converter 100r. The LED light source 502 is an LED string, and includes a plurality of LEDs connected in series. The rectifying circuit 504 and the smoothing condenser 506 rectify and smooth a commercial alternating current (AC) voltage VAC to convert it into a DC voltage VDC. The switching converter 100r receives the DC voltage VDC as an input voltage VIN and supplies a driving current ILED corresponding to a desired amount of light to the LED light source 502.
The switching converter 100r is a step-down converter, and includes an output circuit 102 and a control circuit 200r. The output circuit 102 includes a smoothing capacitor C1, a rectifying diode D1, a switching transistor M1, an inductor L1, a detection resistor RCS, a capacitor C11, and resistors R11 and R12.
During an ON period of the switching transistor M1, a coil current IL1 flows to the detection resistor RCS by way of the inductor L1 and the switching transistor M1. A voltage drop (current detection signal) VCS of the detection resistor RCS is fed back to a current detection (CS) terminal of the control circuit 200r. The control circuit 200r generates a pulse signal whose duty ratio is adjusted such that a peak of the current detection signal VCS becomes close to a target voltage corresponding to a target amount of light of the LED light source 502, and switches the switching transistor M1 connected to an OUT terminal. A driving current ILED smoothed from the coil current IL1 is supplied to the LED light source 502.
A zero cross detection signal VZT that is based on a drain voltage VD of the switching transistor M1 is input to a zero cross detection (ZT) terminal of the control circuit 200r through the capacitor C11 and the resistors R11 and R12. The control circuit 200r detects that the coil current IL flowing through the inductor L1 becomes zero (zero cross) according to the zero cross detection signal VZT, and turns on the switching transistor M1. Further, the control circuit 200r detects that the current detection signal VCS reaches a target voltage VREF, and turns off the switching transistor M1.
A DC voltage VDC is input to a high voltage (VH) terminal of the control circuit 200r, and a capacitor C21 is connected to a power source (VCC) terminal. The control circuit 200r includes a start-up circuit (not shown in FIG. 1, 202 of FIG. 4) between the VH terminal and the VCC terminal. The start-up circuit charges the capacitor C21 when the switching converter 100r is started up.
When a user of the lighting device 500r turns on a switch for lighting on and off the lighting device 500r, the AC voltage VAC is supplied to the rectifying circuit 504 and the voltage VDC of the smoothing condenser 506 increases. In response to turning on the switch, the control circuit 200r is started up. When the control circuit is started up, a start-up current flows from the VH terminal to the capacitor C21 by way of the start-up circuit 202 and the VCC terminal so that the capacitor C21 is charged.
A voltage VCC of the capacitor C21 is a power source voltage of the control circuit 200r. The control circuit 200r is operable when the power source voltage VCC exceeds a predetermined threshold voltage (lowest operating voltage) VUVLO, and starts to switch the switching transistor M1.
When the LED light source 502 has a high temperature, a lifespan of the LED light source 502 may be shortened or the reliability of peripheral circuit components may be degraded. Thus, a temperature of the lighting device 500r is monitored using a temperature detection element such as a thermistor, and an overheat protecting function for decreasing the driving current ILED supplied to the LED light source 502 when the temperature of the lighting device 500r increases, and suppressing an additional increase in the temperature is provided. In particular, in a closed type lighting device that is used in an outdoor area, a bathroom, or the like, since heat is easily generated in terms of structure of the lighting device, it is particularly important to consider a temperature.
A negative temperature coefficient (NTC) terminal is installed in the control circuit 200r. A thermistor RNTC is connected between the NTC terminal and a ground terminal. Further, a capacitor CNTC is installed in parallel to the thermistor RNTC. At the NTC terminal, a voltage corresponding to a temperature is generated.
For example, the control circuit 200r includes a bias circuit 204 connected to the NTC terminal, and supplies a constant current IC to the thermistor RNTC. A voltage (temperature detection voltage) VNTC of the NTC terminal is given as a following equation:VNTC=RNTC×IC 
The thermistor RNTC has negative temperature characteristics, and a resistance value of the thermistor RNTC decreases as a temperature increases. In other words, the temperature detection signal VNTC is decreased as a temperature increases. In the control circuit 200r, the temperature detection signal VNTC suppresses the driving current ILED according to the terminal voltage VNTC in a region where an ambient temperature Ta is higher than a predetermined threshold temperature TTH.
The present inventor reviewed the switching converter 100r of FIG. 1 and recognized the following technical problem. FIG. 2A is a view illustrating a temperature dependency of the temperature detection signal VNTC, and FIG. 2B is a view illustrating a temperature dependency of the driving current ILED. In a range in which the ambient temperature Ta is lower than the threshold temperature TTH, a current is not limited and the driving current ILED maintains a target current IREF (128 mA). When the ambient temperature Ta exceeds the threshold temperature TTH, the driving current ILED is clamped according to the current detection signal VNTC and the driving current ILED decreases as a temperature increases.
Here, in the region of Ta>TTH, when the ambient temperature Ta is slightly changed, the driving current ILED is significantly changed. This means that it is impossible to control an amount of current of the driving current ILED, that is, an amount of light of the LED light source 502, in an overheated state, which is not desirable.
Herein, in order to clarify the technical problem, the lighting device 500r having the LED light source 502 has been described as an example, but in some cases, it may also be intended to set an electrical state of a load in an overheated state or in another abnormal state, even for the switching converter 100r for supplying power to a certain load in applications other than the lighting device 500r. 